1. Field of the Invention
This invention relates to a photoelectric conversion device having a semiconductor region for accumulating carriers generated by photoexcitation, which device reads the signals according to the accumulated carriers in said semiconductor region.
2. Related Background Art
FIG. 1(A) is a schematic sectional view of one example of the photoelectric conversion device of the prior art, and FIG. 1(B) is its equivalent circuit diagram.
In each Figure, photoelectric conversion cells are arranged on n silicon substrate 101, and each cell is electrically insulated from adjacent cells with an element separating device (not shown) consisting of Si0.sub.2, Si.sub.3 N.sub.4 or polysilicon, etc.
Each cell has the constitution as described below.
In the n.sup.- region 102 with low impurity concentration formed by epitaxial technique, etc., a p base region 103 and a n.sup.+ emitter region 104 are formed to constitute a bipolar transistor. Further, on the p base region 103, a capacitor electrode 106 is formed with an oxide film 105 sandwiched therebetween to constitute a capacitor Cox for controlling base potential as opposed to the p base region.
Also, p.sup.+ regions 107 and 108, and a gate electrode with the oxide film 105 sandwiched therebetween are formed to constitute a PMOS transistor for performing refresh actuators.
Otherwise, there are formed an emitter electrode 110 connected to the n.sup.+ emitter region 104, an electrode 111 connected to the p.sup.+ region 108, and a collector electrode 112 on the back of the substrate 110 with an ohmic contact layer sandwiched therebetween, respectively.
Next, the actuation of the above photoelectric conversion device will now be described.
Light is incident from the side of the p base region 103. Carriers (here holes) corresponding to the dose of incident light are accumulated in the p base region (accumulated actuation).
The base potential is changed by the carriers accumulated, and by reading its potential change from the emitter electrode 110, electrical signals corresponding to the incident dose can be obtained. Specifically, the emitter electrode 110 is maintained under floating state with a positive voltage being applied on the capacitor electrode 106. By this, the base potential is elevated to apply 1 bias in the ordinary direction between the base the emitter, and the accumulated voltage in the base being read from the emitter side (reading actuation). Even when the reading actuation may be completed, since the accumulated carriers in the p base region are not substantially reduced, the same signal can be read repeatedly (non-destructive reading).
To perform a refresh actuation which eliminates the holes accumulated in the p base region 103, the emitter electrode 110 is grounded and simultaneously the electrode 111 is maintained at a constant potential.
At first, a negative voltage is applied on the gate electrode 109 to turn on the pMOS transistor Qc. As a result the potential in the p base region 103 becomes a constant value regardless of whether the accumulated potential is high or low.
Subsequently, by applying a positive pulse for refresh on the capacitor electrode 106, a bias is applied in the ordinary direction between the base and the emitter as a result the accumulated holes are eliminated through the grounded electrode 110. When the refresh pulse has risen, the p base region 103 is returned to the initial state of a negative potential (refresh actuation).
Thus, after the potential in the p base region 103 is made at a constant potential by the MOS transistor Qc, a refresh pulse is applied to erase the residual charges, and therefore fresh accumulation can be effected without dependence on the accumulated potential of the previous time. Also, the residual charges can be extinguished rapidly, whereby high speed actuation is rendered possible.
Thereafter, the respective actuations of accumulation, reading, refresh are similarly repeated.
The capacitor Cox may be sometimes not required to be provided, and photoelectric conversion reading can be performed also in the case of a photoelectric conversion cell in which only the transistor Qc is electrically connected to the base region 103.
However, the photoelectric conversion device of the prior art as described above has the following problems.
The surface of the base region 103 has a relatively lower impurity concentration for realizing a high Hfe, and therefore was susceptible to the influence from the potential on the cell surface through the oxide film 105 to be unstable.
Further, the surface recombination current is generated from the silicon surface, and this current is greatly dependent on the surface potential. For this reason, in the photoelectric conversion device having a base region 103 with unstable surface, dark current caused by the above surface recombination current is greatly different between the cells to become a cause for noise.